Current production hydraulically controlled transmission systems normally include a single hydraulic pump driven mechanically by the engine. This single pump provides hydraulic fluid to all fluid consuming subsystems, such as the hydraulic transmission control valve circuit and the transmission lubrication circuit. This single pump will deliver hydraulic fluid at the highest pressure required by any one of the subsystems. This pressure may exceed the pressure required for some of the subsystems. Thus, excessive power loss is caused by pumping oil at higher flow and higher pressure than necessary to fulfill sub-system requirements. The mechanically driven transmission pump provides a flow rate proportional to engine speed, rather than the flow required by the subsystem.
U.S. Pat. No. 7,401,465 was issued July 2008 to Emmert et al, and is assigned to the assignee of the present application. The system shown in the '465 patent includes two pumps, one pump operating at 30 bar to fill the hydros for an IVT, and one pump operating at 20 bar for clutches which flows over a pressure regulating valve to the cooler and lubrication circuit. The higher pressure pump cascades oil into the lower pressure pump circuit (only downhill flow). In this system fluid cannot flow from the low pressure pump into the higher pressure circuit. It is desired to have a two pump system wherein the lower pressure pump can be commanded to provide flow into the high pressure circuit to assist the higher pressure pump.